The present invention relates generally to the operation of rectifier bridges and more specifically to a scheme for the prevention of or guarding against commutation faults, particularly those which are occasioned by a too rapid retardation in the normal firing angle of the controlled rectifiers.
In any polyphase rectifier system, each phase carries all of the current for a portion of the time. For example, in a three phase circuit, each phase carries all the current for one-third of the time. With diodes, transfer or commutation from phase to phase is automatically achieved by the next phase having a greater instantaneous voltage than the phase which is then carrying current. When controlled rectifiers, such as thyristors (e.g., silicon controlled rectifiers) are used, however, the load voltage is controlled by delaying the transfer of current between the phases. This is commonly known as phase control. While phase control is a proven and effective method of controlling the load voltage, care must be taken in order that the voltage-time relationships are sufficient to allow the commutation or transfer so that a commutation failure resulting in an undesirable condition does not occur.
When the alternating current (a.c.) source supplying the rectifier bridge has significant inductance, the transfer of the current or the commutation from phase to phase takes both voltage and time. There must be sufficient volt-seconds available to force the current off in one phase and to bring it up in the next phase. For any particular source, the volt-seconds required are a function of the current to be commutated and not the point at which transfer is initiated. A typical system may require, for example, forty-five electrical degrees to achieve commutation at full phase conduction.
When the rectifiers are driving a highly inductive load, such as a generator field having a time constant measured in seconds, the current to be transferred immediately following a sudden output voltage reduction is the same as it was prior to the sudden reduction and the volt-seconds required for commutation are the same. If the initiation of transfer is delayed too long by the controlled rectifiers, the remaining volt-seconds available may be insufficient to complete the transfer and a commutation failure will result. Similarly, when the bridge is used to drive a highly inductive load such as a motor load, if the demand on the motor is suddenly reduced such that the normal control governing the firing of the rectifiers is phased back or retarded very rapidly, the same situation concerning the available volt-seconds and the current could exist.
As the load current decreases in response to the lower voltage requirement, fewer volt-seconds are required to complete the current transfer and the firing of the rectifier, the initiation of current transfer, may be further delayed without resulting in an improper operation.
One early method of operation of the bridges to insure proper commutation was to place what is commonly known as a free-wheeling diode across or in parallel with the load. This was a satisfactory solution for the technology of the past since the semiconductors then used could not be operated at high current densities for thermal reasons. As such, the forward voltage drop at maximum current was close enough to the voltage drop at the current level where the controlled rectifier could gain control such that a diode placed in parallel with the two series connected cells of the bridge would divert the load current and allow the controlled rectifiers of the bridge to regain control. With modern semiconductors, operation at a much higher current density is quite common and this is no longer true. In the present situation, if a bridge having a free-wheeling diode is suddently phased back from full rated current to some very low desired value, the free-wheeling diode will have too large a forward voltage drop to divert all the current and the current will divide between the free-wheeling path and the bridge path such that the current through the bridge path will be more than enough to keep the controlled rectifier (the bridge path) conductive. Thus, a commutation failure will result.